DEF for Generators — Top 10 Questions
Published in
3 min readJul 11, 2018
- What is DEF. Diesel Exhaust Fluid (DEF) is a mixture of Urea in De-ionized water in a 32.5 % concentration. It is used as a consumable fluid in diesel engine emissions control using Selective Catalytic Reduction (SCR).
- How Much DEF do I Need. A good rule of thumb for generators is that DEF consumption will be about 10% of diesel consumption. Tanks can be sized at 10% of the fuel storage requirements. Pump and pipe systems for replenishment can be sized as well for about 10% of the diesel systems.
- Is DEF required for New Generators. Emissions from generators are now a matter of law. Various environmental agencies have ruled that generators must stop if their SCR emissions systems are not performing to legal requirements. DEF systems for generators should follow the same critical design considerations, including redundancy, that is applied to generator fuel and other systems.
- Is DEF Corrosive. It’s the de-ionized water that makes it corrosive. Pumps, valves, tanks, piping, and sensors should all be stainless steel construction. Plastics and fiberglass may be suitable for some applications as well.
- What are low Temperature Issues? DEF Freezes at 12F ( -11C). The freeze point of DEF is 12F (-11C). It can be stored inside within heated space and cubic tanks can help you save space. Outside it should be in insulated tanks and piping with heaters.
- What are High Temperature Problems. DEF Degrades at 85F (30C). DEF degrades at high temperatures. Limiting high temperature to less than 85F (30C) will assure a minimum of 12 months shelf life. DEF should be stored in insulated tanks in cool and shaded spaces. In warmer climates consider a DEF chiller system to assure quality and save money on replacement.
- How is DEF Tested. There is a Narrow Quality Band +/- 2% Concentration. The ISO DEF Specification allows a range of 31.8 to 33.2 % concentration in de-ionized water. There are simple test devices for DEF concentration that can be used. DEF sensors / analyzer units (Earthsafe C71) are also available for continuous monitoring and integration.
- Where is the Interface to the SCR. The generator SCR systems are certified to environmental standards as a system that includes the engine, SCR unit, and the DEF Dosing equipment. The Dosing “system” includes the DEF Dosing Pumps, Pressure Regulation, Sensors, Controls, and sometimes a buffer tank. The DEF storage and Supply system interfaces the generator equipment at the Dosing Skid or Cabinet, or the Dosing Buffer Tank.
- Are Special Pump and Pipe Schematics Needed. DEF piping systems are usually configured as a loop from the bulk storage tank to the generators. The return line at the tank includes a back pressure regulating valve for pressure control. The piping layout allows DEF to be circulated so that DEF standing in the pipe does not de-grade. Pump systems are usually setup to continuously circulate, or at least while generators are operating. If not operated continuously pumps should circulate DEF in the piping for a few cycles at least once per week.
- How is DEF Integrated and Monitored. Like any critical system generator DEF systems should be continuously monitored and integrated into BMS / SCADA systems. In some locations monitoring for low level in DEF storage tanks may be required by regulation to be integrated into generator shutdown schemes.
